1. Field of the Invention
The present invention relates to a method, an apparatus, and a computer program of processing an image, and more particularly to a technique of reducing image deterioration when erroneous determination occurs in image area separation.
2. Description of the Related Art
For the purpose of improving quality of photocopied images, an image processing method using image area separation has already been known in which process to be performed is switched according to characteristics of an image such as characters, pictures and patterns, or backgrounds (for example, Japanese Laid-open Patent Publication Nos. 2003-101773, 10-173916, 2004-272902, 07-264409, and 2003-046772). However, in the conventional image processing method using the image area separation, for a low light intensity image (low signal-to-noise (S/N) image), erroneous determination in the image area separation have often occurred because of noise from a reading device (a portion of a picture is erroneously determined as a text area). In other words, in a low-light-intensity scanner device aiming for energy saving, there has been a problem that process switching with erroneous determination in the image area separation greatly influences deterioration in image quality.
Moreover, if a method without using the image area separation is adopted, when chromatic aberration occurs while reading characters in black, because it is difficult to determine whether it is the chromatic aberration of black characters just by detecting a local edge amount, the amount of CMY cannot be reduced. As a result, characters are to be black characters with colors, causing such a problem that the characters appear blurry.
In Japanese Laid-open Patent Publication No. 2003-101773, an image processing apparatus aiming for improvement in image quality is disclosed. The apparatus detects an edge region in binary data and an edge amount in multiple-value data for an input image, and detects a third characteristic amount by selectively switching those data. By controlling image correction in a subsequent stage using this result, deterioration in image quality such as “crack” and “coloring” of an edge portion of characters can be handled.
In Japanese Laid-open Patent Publication No. 2003-101773, use of a separation determination result in binary data and an edge amount in multiple-value data in combination for image correction in a subsequent stage is disclosed. However, in this technique, because the edge region and the edge amount are both detected from an input image, deterioration in image quality cannot be handled that occurs when erroneous determination of an edge region is made for an input image with a low S/N as described above. The portion determined as an edge is maximum-valued regardless of a value of the edge amount.
In Japanese Laid-open Patent Publication No. 10-173916 also, an image processing apparatus having means of performing image areas separation is disclosed. In Japanese Laid-open Patent Publication No. 10-173916, an image processing apparatus that has a first text/image-separation determining unit that functions as a first determining unit, a first image correcting unit that performs control based on a result thereof, an edge determining unit that functions as a second determining unit, and a second correcting unit that performs control based on a result thereof is disclosed for the purpose of saving thin multi-sided characters of small point sizes. Furthermore, as for the edge determining unit, a component positioned subsequent to the first image-quality correcting unit is also described.
Although the technique disclosed in Japanese Laid-open Patent Publication No. 10-173916 is a technique thus having the determining units in two stages, it is to save, by the second determining unit, text that cannot be separated by the first determining unit, and it is considered that this is not to solve the problem of deterioration in image quality when a portion in an image is erroneously determined as text by the first determining unit.
In Japanese Laid-open Patent Publication No. 2004-272902, description about image processing in which image-area separation determination is performed, and image quality correction is performed using a result of the determination and a result of an edge amount after smoothing processing in combination is considered to be included. In Japanese Laid-open Patent Publication No. 07-264409, description that image area separation is performed, and image processing of removing a background is performed is considered to be included. In Japanese Laid-open Patent Publication No. 2003-046772, a specific method for image-area separation process is described.
Embodiments disclosed in this specification, to support low light-intensity scanners, image processing so as to suppress deterioration in image quality even when erroneous determination is made in the image area separation is performed. More specifically, an image characteristic amount is extracted after determination in the image area separation and smoothing filtering process are performed, and image-quality correction processing (black generation, under color removal) in a subsequent stage is performed using those results in combination.
When an amount of under color removal is calculated in continuous values to perform correction for removal at the time of under color removal process, some amount of memory for operation required for the process is necessary. However, even if the amount of under color removal is calculated in excessively continuous values, it does not actually have great influence on the image quality.
In view of the above situations, there is a need to perform image processing that enables to suppress deterioration in image quality at low cost even when erroneous determination occurs in image area separation.